Hi @Ludakit , and welcome to the Community!
There's no need to worry about the information you've provided, and it's great to see that you've already looked into more detail as to what might have caused this.
In this instance, can I ask that you check the following for me please?
1) Please double check on the receiver boxes that these are set to L and R respectively, and that they have not both been set to the same channel by accident!
As you are facing the TV/Soundbar from where you are sat, the one to your left should be L, and the one to the right should be R.
2) When you say that this recognises the rear speakers, I presume that this is displaying a solid white LED on both receiver boxes, as well as the Acoustimass 300 Bass Module. (If this is not the case, please let me know.) If so, please perform the factory reset to the Soundbar, then pair these back up.
Before performing the factory reset, disconnect all the accessory speakers (Surround Speakers and Acoustimass 300 Bass Module) from power. We can connect these back up later.
The factory reset can be performed by first pressing the SoundTouch button on the remote control (directly below "Source power"), then pressing and holding the "Teletext" button (to the right of the CC button on the remote) for 10 seconds until the Wi-Fi, TV, SoundTouch and Bluetooth indicators blink white on the soundbar.
This will reset the Soundbar, which takes roughly 1-3 minutes. When reset, the Wi-Fi indicator on the far Left will glow solid amber.
When reset, please perform the connection method using ST and 7 as you have before until the connectivity indicator on the soundbar blinks white. Then connect up both surround speakers to power, and confirm that these connect. Once connected, repeat the ST and 7 process but with the Acoustimass 300 Bass Module instead so that this is paired up as well, then test this audio again!
3) Please also check that you have the source audio set to 5.1 Surround Sound, and that this is not set to something with less channels, such as stereo/mono sound as this would limit what you hear from these speakers!
I would ask that you get back to me when you have an opportunity to do so to let me know how the above goes!
Liam W - Community Support
A few more things you could try:
Log out of your SoundTouch account then log back in.
Reset your TV
Disconnect and reconnect your optical cable
Make sure your TV’s optical output is set to 5.1
Find and use a source like Netflix to ensure DD 5.1.